1. Field of the Disclosure
This invention pertains to a photosensitive element and a method for preparing a printing form from the element, and in particular to a photosensitive element having a photopolymerizable composition containing an additive and a method for treating the element to form a surface suitable for printing.
2. Description of Related Art
Flexographic printing plates are well known for use in printing surfaces which range from soft and easy to deform to relatively hard, such as packaging materials, e.g., cardboard, plastic films, aluminum foils, etc. Flexographic printing plates can be prepared from photosensitive elements containing photopolymerizable compositions, such as those described in U.S. Pat. Nos. 4,323,637 and 4,427759. The photopolymerizable compositions generally comprise an elastomeric binder, at least one monomer and a photoinitiator. Photosensitive elements generally have a photopolymerizable elastomeric layer interposed between a support and a coversheet or multilayer cover element. Upon imagewise exposure to actinic radiation, photopolymerization of the photopolymerizable layer occurs in the exposed areas, thereby curing and rendering insoluble the exposed areas of the layer. The element is treated with a suitable solution, e.g., solvent or aqueous-based washout, or thermally, to remove the unexposed areas of the photopolymerizable layer leaving a printing relief which can be used for flexographic printing.
In some instances it may be desirable to directly engrave the printing form with laser radiation of sufficient intensity to ablate the elastomeric material and form a surface, e.g., relief, suitable for printing. Photopolymerizable printing elements are often photochemically reinforced by overall exposing the element to actinic radiation before imagewise ablating with laser radiation. Particulate material that is sensitive to the laser radiation may be added to the photosensitive composition in order to enhance the engraving efficiency of the element and to reinforce the mechanical properties of the element.
U.S. Pat. Nos. 5,798,202 and 5,804,353 disclose process for make a flexographic printing plate by laser engraving one or multiply reinforced elastomeric layers on a flexible support. The process involves reinforcing and laser engraving the one or multiple reinforced elastomeric layers. The elastomeric layer can be reinforced mechanically, or photochemically, or thermochemically, or combinations thereof. Mechanical reinforcement is provided by incorporating reinforcing agents, such as finely divided particulate material, into the elastomeric layer. Carbon black and graphite are cited as suitable reinforcing agents. Photochemical reinforcement is accomplished by incorporating photohardenable materials into the elastomeric layer and exposing the layer to actinic radiation. Photohardenable materials include photocrosslinkable and photopolymerizable systems having a photoinitiator or photoinitiator system. Both patents provide examples of photosensitive elements that are only mechanically reinforced with carbon black, in which the amount of carbon black in the elastomeric layer was from 1 to 25% by weight of the elastomeric layer. Both patents also provide examples of photosensitive elements that are both reinforced mechanically with carbon black in the elastomeric layer and reinforced photochemically. However, in U.S. Pat. No. 5,798,202 the amount of the reinforcing agent in the elastomeric layer was considerably less (compared to elastomeric layers that were only mechanically reinforced) in order to photochemically reinforce the elastomeric layer which had 2-phenyl-2,2-dimethoxy acetophenone (which is a derivative of acetophenone) as a photoinitiator. The amount of carbon black in the photosensitive elements of the examples where the elastomeric layer was mechanically and photochemically reinforced was less than 0.23% by weight of the elastomeric layer, so that the actinic radiation could adequately penetrate the layer and photochemically reinforce the element. An example of U.S. Pat. No. 5,804,353 prepared a multilayer plate in which a top layer of a photosensitive composition included carbon black as mechanical reinforcing particles at 18.7% by weight, and 2-isopropylthioxanthone and ethyl-p-dimethylaminobenzoate as a photoinitiator system. However, the multilayer plate was laser engraved for use as a flexographic printing plate.
Flexographic printing is a form of relief printing in which the printing form prints from an image area, where the image area of the printing form is raised and the non-image area is depressed. Gravure printing is a method of printing in which the printing form prints from an image area, where the image area is depressed and consists of small recessed cups or wells to contain the ink or printing material, and the non-image area is the surface of the form. However, gravure printing forms are expensive and require considerable time and material to produce. A gravure cylinder is essentially made by electroplating a copper or chrome layer onto a base roller, and then engraving the image composed of the small recessed cells or wells digitally by a diamond tipped or laser etching machine.
It is contemplated that the photopolymerizable relief printing form could be used for gravure-like printing. Gravure-like printing is similar to gravure printing except that a relief printing form is used wherein the image area is depressed and consists of recesses areas forming wells to carry the ink which transfer during printing. However, the mechanical properties, such as abrasion resistance, tensile strength, and stiffness of the photopolymerizable relief printing form would need to be enhanced in order for the element to effectively function as or similar to a conventional gravure cylinder. It is expected that the recessed areas of the elastomeric flexographic printing form that carry the ink could change volume during ink transfer or nip contact with the substrate rendering unacceptable image quality. Also, conventional gravure printing typically uses a doctor blade to regulate the amount of ink transferred to the recessed areas of a gravure cylinder. But the elastomeric flexographic printing form in gravure-like application is not likely sufficiently resistant to the abrasion and wear associated with continuous contact of the doctor blade. Hereto, particulate material may be added to the photosensitive composition in order to reinforce the mechanical properties of the element. A photopolymerizable printing form functioning for gravure-like printing could be easily imaged by laser engraving or by conventional imagewise exposure and treatment as described previously.
Additionally, the solvent resistance of the photopolymerizable relief printing form also needs to be enhanced in order for the printing form to effectively function as or similar to a conventional gravure cylinder. Most inks used in conventional gravure printing typically contain toluene, which is a potent solvent. An elastomeric flexographic printing form is not sufficiently resistant to toluene or other strong solvents, and may dissolve or swell upon contact with the ink, particularly over time. Swelling of the printing form on press will result in poor quality of the printed images. It is also possible that the solvent ink may cause some of the components from the printing for to leach out, which could cause complete failure of the printing form on press.
A problem associated with the photosensitive printing forms having an elastomeric layer which is reinforced both mechanically and photochemically is that laser engraving does not efficiently remove the elastomeric material to provide desired relief quality, and ultimately printing quality. Photochemically reinforcing the element can provide the desired properties for engraving as well as in its end-use as a printing plate. However, the presence of a particulate additive, like a reinforcing agent, tends to reduce the penetration of the ultraviolet radiation required to photochemically reinforce the element. If the elastomeric layer is insufficiently cured during photochemical reinforcement, the laser radiation cannot effectively remove the material and poor relief quality of the engraved area results. Further, the debris resulting from laser engraving tends to be tacky and is difficult to completely remove from the element. Additionally, if the element is not sufficiently photochemically reinforced the required end-use properties as a flexographic or gravure-like printing plate are not achieved. These problems tend to be exacerbated with increasing concentration of the additive that enhances engraving efficacy or the mechanical properties for gravure-like printing.